Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In 1880 Thomas Edison invented the light bulb using a carbon filament. Edison's light bulb provided 40 hours of light in an oxygen-free environment ushering in the electric lighting era. The carbon filament, however, has low reliability at high operating temperatures. To replace the carbon filament, more than 40 elements have been tested as a filament material. In 1910, William D. Coolidge successfully substituted a tungsten filament in light bulbs for Edison's carbon filament.
Incandescent light bulbs, however, provide light via blackbody (thermal generated) radiation. The visible light of a typical vacuum tungsten light bulb is approximately 5% of the total radiation. Thus, a majority of electrical energy to operate the bulb is converted to heat. Increasing the operating temperature increases efficiency, however, this method of increasing the efficiency is limited by the melting temperature of tungsten.
Sodium lamps are more efficient than tungsten lamps, but their light is essentially monochromatic and not pleasing to the human eye. Light-emitting diodes are not widely used currently because of their complicated fabrication technique and high processing cost.